Scientist and engineers have been exploring many different types of fuel injector systems for gas turbine engines in order to meet cycle goals for advanced engines and particularly for future gas turbine engines that are capable of performing at ever increasing overall fuel loading conditions during high-power operations. Because the high fuel loadings at high power conditions dictate an extreme fuel/air ratio at the front end of the combustor these systems have a high propensity for producing smoke. These high fuel/air ratio conditions obviously produce an undesirable amount of soot that needs be eliminated or minimized in one way or another. One method of minimizing soot formation is to oxidize this primary zone soot to acceptable levels and in order to accomplish this feat an increase in the length of the intermediate zone of the combustor is necessary. The increased length, obviously, increases the overall combustor length with a consequence in increased combustor and engine size and a corresponding increase in weight. The increased size and weight in light of future aircraft requirements are intolerable conditions that need to be avoided in order to assure that the engine meets certain thrust to weight specification and of course, meet engine performance requirements.
U.S. Pat. No. 5,603,211 granted to Graves on Feb. 18, 1997 entitled "Outer Shear Layer Swirl Mixer For A Combustor" exemplifies a system that attempts to resolve the high fuel loading conditions at high power by providing localized high front-end fuel/air ratios while attempting to reduce smoke and maintaining or improving on the flame relight stability of the combustor. Other examples of concepts designed for the same purpose are disclosed in U.S. patent application Ser. No. 08/947,554 filed by Graves et al, entitled Fuel Injector For Gas Turbine Engine and Ser. No. 08/947,593 filed by Graves both on Oct., 9, 1997 and all being commonly assigned to United Technologies Corporation, the Assignee of this patent application. All these references are incorporated herein by reference and should be referred to get a better understanding of the details of the fuel nozzle and mixers that are being considered in this application.
As one skilled in this field of technology recognizes, in order to address the problems of additional length, weight and smoke as presented in these prior art systems the designer of the combustor is moved to design the combustor to include as much injector air in the front end of the combustor as could be tolerated. These high shear swirlers as presented in these referenced prior art patent and patent applications include an outer annulus shear zone to atomize the fuel. Such injectors have been developed with effective air flow areas (ACd) that are as high as 0.80 square inches. As noted FIGS. 2a, 2b, 2c and 2d, which are a series of graphs, demonstrate the effect that ACd injector has on smoke, nitrous oxide (Nox), Pattern Factor and Flame Stability. It will be appreciate that overall smoke levels, exit temperature pattern factor and nitrous oxide emissions decrease significantly as ACd rises. However, increased injector air at the front end of the combustor also produces a flame that is inherently prone to flame blowout at low power conditions.
As understood by those skilled in this art, these prior art systems exemplified by the referenced patent and patent applications, supra, the solution to the idle stability problem and the desired high power performance introduced a dilemma. One solution to this dilemma is to use multi-zones having a low power fuel nozzle to enhance the stability of the combustor. These multi-zone fuel injection systems, as these prior art systems have become to be known as, created a relatively rich burning region in the "pilot zone" of the combustor which was needed to provide a good idle stability margin. At high power conditions, the secondary zone or "main" zone in the combustor is fueled to prevent the formation of excessive smoke and Nox. When operating in a dual-zone mode, as is the case when a pilot zone and main zone are utilized, these burners also provide good exit temperature pattern factor. Unfortunately, these dual zone systems introduced complexity in the overall fuel systems. The requirement of the additional set of fuel nozzles in these dual zone systems also increased the weight of the fuel injection system and hence, results in a deficit to the overall engine performance. In comparison with a single-zone conventional combustor the multi-zone fuel injection systems not only introduced complexity in the overall burner or combustor, it also increased its weight.
I have found that I can obviate the complexity and weight problems discussed in the above paragraphs by providing an injector that consists of two assemblies, namely, 1) a burner-mounted swirler with two outer air passages surrounding a concentric ring of air injection holes and (2) a piloted fuel nozzle containing an airblast-atomized main (or secondary) fuel injection annulus and a pilot (or primary) fuel injection orifice, hereinafter referred to for convenience and simplicity as a Multishear Injector. The primary passage of the Multishear Injector provides low-power fuel to the external portions of the injector, promoting good ignition performance and robust stability. The secondary passage of the Multishear Injector provides high-power fuel to the central regions of the injector through an annular fuel injection passage surrounded by concentric swirled air passages. This main fuel assembly provides good atomization and a uniform fuel spray, thus, reducing Nox emissions without incurring a significant increase in smoke. In accordance with this invention the fuel nozzle mounts into the swirler upon final burner assembly when the nozzle pilot tip is accepted through the swirler assembly at a single bearing location.
Results of experimental testing has demonstrated that the preferred embodiment of the Multishear Injector significantly reduced Nox emissions (below the current low-NOx combustors), equivalent idle stability to conventional burners, and reduced smoke emissions relative to all heretofore known systems.
The inventive Multishear injector provides all the benefits of a fully staged burner, good idle stability, lean-blowout stability, low idle emissions, low mid-power emissions, low smoke, low high-power Nox emissions and good altitude lighting, without the associated increase in burner complexity and weight necessary to support integration of two separate fuel injection zones. Also, the external fuel system architecture can be identical to current unstaged burners, which is another significant weight, cost and durability improvement of heretofore known systems.